Various internal combustion engines which can be classified as segregating engines invented by the applicant are known, for example from GB-A-2155546, GB-A-2186913, GB-A-2218153, GB-A-2218157, GB-A-2246394 and GB-A-2261028. Those engines are now known in the literature as the Merritt Engines.
The Merritt engine comprises at least one set of first and second cylinders and respective first and second pistons movable in said cylinders, in each set, the first cylinder having a larger swept volume than the second cylinder and there being an air inlet valve and/or port communicating with the first cylinder, an exhaust valve and/or port communicating with the first cylinder a fuel source for providing fuel to the second cylinder, means defining combustion space when the pistons are substantially at the inner dead centre position, the combustion space communicating with both cylinders during at least part of the expansion stroke, and inhibiting means for inhibiting ingression.
The term "air" as used herein includes any suitable mixture of oxygen with other usually inert gases as well as substantially pure oxygen for combustion with gaseous or liquid (i.e. vaporised liquid) fuel. It may contain recirculated exhaust gases, crankcase gases and a small proportion of hydrocarbon substances present in recirculated internal combustion engine gases.
The term "ingression" as used herein refers to the movement of fuel/air mixture from the second cylinder into the combustion space.
The Merritt engine is a segregating engine like the diesel engine with a difference that some small quantity of air is compressed with substantially all the fuel in the smaller second cylinder, whereas most of the air is compressed on its own in the larger first cylinder.
An important characteristic of segregating engines such as the diesel engine and the Merritt engine is the confinement of the fuel, away from most of the air, during most of the compression stroke of the engine. The Merritt Engine does so by using a smaller cylinder and piston which receives the fuel during the induction stroke and segregates it from the bulk of the air until the moment of ingression, near the end of the compression stroke. The smaller cylinder can be referred to as the fuel management cylinder.
A segregating engine is eminently suitable to use the process known as compression-ignition to ignite the fuel the since fuel is not mixed with enough air to ignite spontaneously during most of the compression stroke, even when high compression ratios are used. In a diesel engine, which is also a segregating engine, the timing of ignition is determined by the timing of fuel injection into the combustion space. In the Merritt Engines so far disclosed the control of ignition timing is effected by controlling the timing of the process of ingression, in other words the transfer of vaporised fuel from the fuel management cylinder into the combustion space. In Merritt engines using high compression ratios, ignition of some of the fuel can take place the moment fuel enters the combustion space and meets with the very hot air therein.
GB-A-2246394 describes a number of ways in which the timing of ingression and hence the timing of ignition can be controlled. In particular, the smaller cylinder is provided with access means to control the pressure in the smaller cylinder to a value below the pressure in the larger cylinder during the early part of the compression stroke, thereby inhibiting ingression prior to the smaller piston arriving at or near its inner dead centre position. The access means described in GB-A-2246394 preferably includes a first port opening into the smaller cylinder. The port may contain a variable flow area valve, or a throttle and a first valve, such as an actuated poppet valve, for controlling access of air and/or fuel through the first port during each cycle of the engine. The fuel source, which may comprise a liquid fuel injector, is preferably arranged upstream of the first valve.
The main advantages of segregating engines, such as the diesel and Merritt engines, is their ability to burn extremely lean overall fuel/air mixtures. The spark ignition engine which has a pre-mixed fuel/air mixture during the compression stroke, is limited to near stoichiometric fuel/air mixtures to allow a flame, initiated with a spark, to move across the whole fuel/air mixture volume in the combustion space. Very lean overall fuel/air mixtures result in an overall cooler expansion process which in turn leads to improved engine thermal efficiency and lower quantities of harmful NO.sub.x gases in the exhaust, particularly at part loads. The thermal efficiency of a reciprocating internal combustion engine rises with lean burn when mean temperatures following heat release fall from the high values encountered in stoichiometric combustion.
The main features for promoting very efficient reciprocating internal combustion engines are very fast combustion and lower gas temperatures following heat release.
The automotive or high speed diesel engine does not achieve fast combustion since, at higher speeds, it is unable to provide sufficient time for the liquid fuel fully to vaporise before it is ignited. On the other hand the diesel engine can promote lower gas temperatures following overall lean burn at part load. The Merritt engine can potentially achieve both faster combustion than the diesel engine under all conditions as well as low part load temperatures. In the Merritt Engine fuel is admitted into the fuel cylinder during the induction stroke and whilst segregated from the main bulk of air the fuel is given much more time to vaporise in a small quantity of air, before the fuel ingresses into the combustion space to ignite.
An example of the known Merritt engine is shown in FIG. 1 of the accompanying drawings which is a partial cross section through a part of the engine, reproduced from GB-A-2246394. This engine is a compression ignition engine which uses a spark plug for igniting the fuel/air mixture during starting and idling in the conventional spark ignition gasoline engine (SIGE) manner where the fuel/air mixture burns as a flame front. The engine is described briefly below and the reader is directed to GB-A-2246394 or corresponding U.S. Pat. No. 5,117,789 for a more detailed description.
The engine comprises a smaller piston 18 mounted on the crown 36 of the larger piston 16. The piston 18 includes a pillar 234 and a crown 35. It will be seen from FIG. 1 that the pillar 234 is curved in contour, the curve encouraging swirl of the air entering the combustion space 20 from the larger cylinder 12 and swirl of the fuel/air mixture following ingression into the combustion space 20. The combustion space is defined between pillar 234 and the wall, indicated generally at 14a, of the smaller cylinder 14. The shape and size of pillar are chosen to produce a suitable combustion volume of appropriate size and shape.
It will be noted that the crown 35 of the piston 18 has an edge with an axial thickness which is substantially less than the axial distance between the crowns 35 and 36 of the pistons 18 and 16. The crown 35 has a cylindrical peripheral edge 37 which is spaced slightly away from the wall 14a of the smaller cylinder to define inhibiting means in the form of an annular gap 128. The upper end of the smaller cylinder 14 as viewed in the drawing is formed with an optional peripheral groove 39 which provides a by-pass to promote ingression as described below. The upper end of the smaller cylinder 14 is provided with the access means comprising the second inlet valve 31 and the throttle valve 32. The fuel injector 34 is provided for delivering liquid fuel into the inlet duct 33. The throttle valve 32 controls the quantity of air flowing through inlet duct 33 and does so substantially independently of the quantity of the fuel delivered by the fuel injector 34.
During the induction stroke of the engine air enters the larger cylinder 12 through the inlet duct 25. Air also enters the smaller cylinder 14 through open valve 31 along with fuel from injector 34. The difference in pressure across the crown 35 of the piston 18 in the early part the compression stroke can be influenced by throttle valve 32 and the timing of the closure of valve 31. This in turn has an effect on the timing of ingression of the contents of the smaller cylinder 14 into the combustion space 20 near the inner dead centre position of the piston 18 towards the end of the compression stroke. Ingression in turn controls the timing of ignition of the vaporised fuel by compression ignition when the fuel/air mixture in cylinder 14 meets the hotter air delivered to the combustion space 20 by the larger piston 16 during the compression stroke.
The groove 39 has an axial length greater than the thickness of the smaller piston crown 35 to provide an enlarged gap for the fuel/air mixture to ingress around the crown through the bypass groove 39. The groove 39 also provides a clearance volume in the smaller cylinder 14 and this clearance volume effectively delays ingression timing by providing extra volume in cylinder 12 during the compression stroke.
The engine shown in FIG. 1 also has a throttle valve 23 Situated in inlet duct 25 which supplies air to the larger cylinder 12, and a spark plug 52. An exhaust valve and exhaust bort are not shown in FIG. 1 but are nevertheless present in the engine in communication with larger cylinder 12. The full line position of the pistons represents the outer dead centre position and the broken lines indicate the pistons at their inner dead centre position.
The purpose of using a spark plug in the known Merritt engine of FIG. 1 is to allow the engine to cope with marginal conditions such as idling and starting. In particular, under low part load conditions the fuel/air ratio in the smaller cylinder could reach a near stoichiometric value ignitable by compression ignition. To avoid this problem, throttle 32 can be partially closed to maintain the fuel/air ratio in the smaller cylinder 14 above the level at which spontaneous compression ignition occurs. However, the reduction in pressure which occurs in the smaller cylinder as a result of this throttling may increase the flow of air from the larger cylinder 12 across the gap 128 into the smaller cylinder 14, thus reducing the fuel/air mixture ratio back towards stoichiometric, and re-establishing the risk of spontaneous compression ignition in the smaller cylinder 14, before ingression commences. To prevent this the throttle valve 23 is used to reduce the compression pressures in the larger cylinder 12 by reducing the air intake to the engine. The effect of this is to reduce the flow of air across the gap 128 into the second cylinder, again removing the risk of spontaneous compression ignition before ingression. The peak compression pressure and temperature also reduce as a result of the air flow reduction by the throttle 23. With reduced end of compression temperatures, the fuel/air mixture may not ignite by compression ignition and the spark plug 52 can then be used to ignite the mixture in the conventional spark ignition manner as the mixture ingresses. The fuel/air mixture then burns as a flame front which is propagated through the mixture in the conventional spark ignited gasoline engine manner.